Such an amplifier biasing circuit is already known in the art and is for instance described in the published European patent application No 0455894 (which corresponds to commonly assigned copending application Ser. No. 07/699,954).
In this known biasing circuit the amplifier is a sense amplifier forming part of a telecommunication ring trip circuit also including a Herter bridge with a first pair of terminals coupled to a telecommunication line, with a second pair of terminals coupled to a ringing signal source and with the terminals of a third pair coupled to respective input terminals of the input of the sense amplifier. The purpose of this sense amplifier is to detect when during a ringing phase, i.e, when a ringing signal is applied to the line by the ringing signal source, a DC line loop becomes closed and to then provide a control signal to switch off the ringing signal, The biasing circuit connected to the input of this sense amplifier prevents the latter from being driven out of its operating range when the common mode voltage provided at its input by the ringing signal becomes too high. To this end the biasing circuit connects the bias impedance to the amplifier input via switches before the ringing signal is applied to it. Otherwise it disconnects this impedance via the same switches. This is correct since in this case the signal at the amplifier input is too small to drive this sense amplifier out of its operating range, thus making the need for biasing the input of the sense amplifier superfluous.
A first drawback of this known biasing circuit is that an outside control signal for driving the switches connecting and disconnecting the bias impedance must be available. A second drawback is that the impedance is varied in an abrupt way from infinite [switches open] in the absence of a ringing signal to a fixed value [switches closed, in the presence of such a signal. Indeed, by proceeding in this way the circuit may be active during a time longer than necessary and may therefore dissipate more power than strictly needed since when the ringing signal has low enough a magnitude in fact no biasing is needed.